1. Field of the Invention
The present invention relates to hydraulic systems for equipment, such as off-road construction and agricultural vehicles, and more particularly to an apparatus for controlling a variable displacement pump used in such systems and for precisely controlling flow of pressurized fluid to hydraulic actuators on the equipment.
2. Description of the Related Art
With reference to FIG. 1, a backhoe-loader 10 is a common type of earth moving equipment that has backhoe assembly 20 attached to the rear of a tractor 15 and a loader assembly 25 mounted at the front of the tractor. The backhoe assembly 20 comprises boozes 12 with one end moveably coupled to the frame of a tractor 15 and another end to which a dipper 13 is pivotally mounted. A bucket 14 is pivotally attached to a remote end of the dipper 13. The bucket 14 can be replaced with other types of work implements. The boom 14 is raised and lowered with respect to the frame of a tractor 15 by a first hydraulic actuator 16. A second hydraulic actuator 17 causes the dipper to pivot at the remote end of the boom. A third hydraulic actuator 18 causes the bucket 14 to tilt with respect to the dipper 13. A joint 21 enables the entire backhoe assembly 20 to swivel left and right with respect to the rear end of the tractor 15, which motion is referred to as “swing” or “slew”. A fourth hydraulic actuator 19 is attached between the frame of the tractor 15 and the boom 12 and provides the drive force for the swinging the backhoe assembly 20.
The loader assembly 25 comprises a load bucket 27 pivotally coupled to the front end of a lift arm 26 that has a rear end pivotally coupled to the tractor 15. A lift hydraulic actuator 28 raises and lowers the lift arm 26 and a load hydraulic actuator 29 pivots the load bucket 27 up and down at the end of the lift arm 26.
In the exemplary backhoe-loader 10, the hydraulic actuators 16-19, 28 and 29 are cylinder-piston assemblies, however, other types of hydraulic actuators, such as a hydraulic motor, can be used in some instances.
The front wheels 24 of the backhoe-loader 10 are steered by another hydraulic actuator, not visible in FIG. 1.
The flow of hydraulic fluid to and from each of the hydraulic actuators 16-19, 28 and 29 is supplied through control valve assemblies that are controlled by a human operator. Each combination of an actuator and a control valve assembly is part of a hydraulic function. The pressurized fluid to drive the hydraulic actuators is supplied by a pump that is driven by the engine 23 on the tractor 15. For greater efficiency, a variable displacement pump often is used to provide the amount of fluid flow required to operate all the hydraulic actuators as commanded at a given time by the operator of the backhoe-loader 10.
Prior hydraulic systems, such as the one described in U.S. Pat. No. 6,098,403, used a load sense (LS) type variable displacement pump. There the pump displacement is controlled by a load sense pressure signal that corresponds to the greatest pressure produced in all the hydraulic actuators in response to the load forces acting on the actuators. As control of hydraulic systems evolved to using computerized controllers, pumps were developed that varied the displacement in response to electrical signals. Such electrically controlled variable displacement control pumps are expensive and not readily available in all capacities and physical sizes required by many types of machines.
Thus there remains a desire to provide a mechanism by which an electrical signal from a hydraulic system controller produces a control signal to vary the flow of a pump.
Another control factor that has to be considered at each function is pressure compensation. Assume that operation of a first function requires supply fluid at a relatively low pressure. In response, the pump is operated to provide the low pressure and the control valve assembly of the hydraulic function is opened accordingly. When a second hydraulic function requiring a significantly greater pressure is activated, the output pressure of the variable displacement pump increases to satisfy the greater pressure demand. The supply fluid at the higher pressure also is applied to the first hydraulic function, which without compensation, results in the flow rate to the hydraulic actuator increasing, thereby resulting in a velocity of the associated hydraulic actuator exceeding the operator's command. To prevent that undesirable effect, the hydraulic functions incorporate a closed loop type pressure compensation valve that responds to a sensed load pressure and the supply pressure. Thus in the above example, the pressure compensation valve reacts to the increase in supply pressure by restricting the fluid flow so that a relatively constant flow occurs to the first hydraulic actuator in spite of supply pressure variation.